The present invention relates to an electroluminescent device and further to a method of manufacturing same. More particularly, the present invention relates to an electroluminescent device in which the light-emitting substance is a semiconductor. Most particularly, the present invention relates to a storable information display screen for use with an electronic information processing device, in which the light-emitting substance is a light emitting polymer (LEP).
Electroluminescent devices are structures which emit light when subject to an applied electric field. The usual model for the physical process in a semiconductor used in this way is through the radiative combination of electron-hole pairs which are injected into the semiconductor from opposite electrodes.
Common examples are light-emitting diodes based on GaP and similar III-V semiconductors.
Although these devices are efficient and are widely used, they are limited in size, and are not easily or economically used in large area displays.
Alternative materials which can be prepared over large areas are known, and among these are the inorganic semiconductors. Most effort has been directed to ZnS.
However, the ZnS system has considerable practical drawbacks, primarily poor reliability. The mechanism in ZnS is believed to be one where acceleration of one type of carrier through the semiconductor under a strong electric field causes local excitation of the semiconductor which relaxes through radiative emission.
Among organic materials, simple aromatic molecules such as anthracene, perylene and coronene are known to show electroluminesence.
Nevertheless, the practical difficulty with these materials is, as with ZnS, their poor reliability, together with difficulties in deposition of the organic layers and the current-injecting electrode layers.
Techniques such as sublimation of the organic material suffer from the disadvantage that the resultant layer is soft, prone to recrystalization, and unable to support high temperature deposition of top-contact layers.
Techniques such as Langmuir-Blodgett film deposition of suitably-modified aromatics suffer from poor film quality, dilution of the active material, and high cost of fabrication.
An electroluminescent device utilizing anthracene is disclosed in U.S. Pat. No. 3,621,321. This device suffers from high power consumption and low luminescence. In an attempt to provide an improved device, U.S. Pat. No. 4,672,205 describes all electroluminescent device having a double layer structure as its luminescent layer. However, the suggested materials for the double layer structure are organic materials which suffer from the disadvantages mentioned above
Light emitting polymers (LEPs) are expected to revolutionize the monitor and computer display, since LEP based display may be manufactured very thin and collapsible, e.g., rollable or foldable.
U.S. Pat. No. 5,399,502, which is incorporated by reference as if fully set forth herein discloses an electroluminescent device which includes (i) a semiconductor layer in a form of a thin dense polymer film comprising at least one conjugated polymer; (ii) a first contact layer in contact with a first surface of the semiconductor layer; and (iii) a second contact layer in contact with a second surface of the semiconductor layer.
The polymer film of the semiconductor layer has a sufficiently low concentration of extrinsic charge carriers, so that on applying an electric field between the first and second contact layers across die semiconductor layer, so as to render the second contact layer positive relative to the first contact layer, charge carriers are injected into the semiconductor layer and radiation is emitted from the semiconductor layer.
U.S. Pat. No. 5,399,502 further discloses a method of manufacturing the above described electroluminescent device which includes steps of depositing a thin layer of a precursor polymer onto a substrate and then heating the precursor polymer to a high temperature to form the conjugated polymer.
The invention disclosed in U.S. Pat. No. 5,399,502 is based on the discovery that semiconductive conjugated polymers can be caused to exhibit electroluminesence by the injection of charge carriers from suitable contact layers.
However, the electroluminescent device described in U.S. Pat. No. 5,399,502 suffers several limitations.
First, being a laminate, the device is expected to be limited in the degree allowed collapsibility, since under high collapse strain, the layers of the laminate are expected to either break or become separated, in either case, functionality will be severely and irreversibly hampered.
Second, the device requires a thick non-luminescent base substrate for mechanical support.
Third, being a laminate, the device is inherently manufactured using e.g., lithography or other complicated procedures, which are, in most cases, to be conducted under special conditions, such as, but not limited to, vacuum, and are therefore cumbersome and cost and effort demanding. Furthermore, the use of lithography limits the maximal size of devices obtainable.
There is thus a widely recognized need for, and it would be highly advantageous to have, an electroluminescent device devoid of the above inherent limitations.